From Strings to Trees to Strings to Trees (Abstract) 
Aravind K. Joshi 
Dept. of Computer and Information Science 
University of Pennsylvania, Philadelphia PA 19104 
Sentences are not just strings of words (or are they 
?), they have some (hierarchical) structure. This much 
is accepted by all grammar formalisms. But how much 
structure is needed? The more the sentences are like 
strings the less the need for structure. 
A certain amount of structure is necessary simply be- 
cause a clause may embed another clause, or one clause 
may attach to another clause or parts of it. Leav- 
ing this need of structure aside, the question then is 
how much structure should a (minimal) clause have? 
Grammar formalisms can differ significantly on this is- 
sue. Minimal clauses can be just strings, or words 
linked by dependencies (dependency trees), or with rich 
phrase structure trees, or with flat (one level) phrase 
structure trees (almost strings) and so on. How much 
hierarchical structure is needed for a minimal clause 
is still an open question, that is being debated heat- 
edly. How are clauses put together? Are these oper- 
ations more like string manipulations (concatenation, 
insertion, or wrapping, for example) or are they more 
like tree transformations (generalized transformations 
of the early transformational grammars, for example)? 
Curiously, the early transformational grammars, al- 
though clearly using tree transformations, actually for- 
mulated the transformations as pseudo string-like op- 
erations! More recent non-transformational grammars 
differ significantly with respect to their use of string 
rewriting or tree rewriting operations. 
Grammar formalisms differ with respect to their 
stringiness or treeness. Also during their evolution, 
they have gone back and forth between string-like and 
tree-like representations, often combining them in dif- 
ferent ways. These swings are a reflection of the com- 
plex interplay between aspects of language structure 
such as constituency, dependency, dominance, locality 
of predicates and their arguments, adjacency, order, 
and discontinuity. We will discuss these issues in an in- 
formal manner, in the context of a range of formalisms. 
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